1. Field of the Invention
The present invention relates to a battery voltage detecting circuit that monitors the respective voltages of at least two batteries which are connected in series.
2. Description of the Related Art
As a known battery voltage detecting circuit that monitors the respective voltages of at least two batteries which are connected in series, there is a battery voltage detecting circuit which is comprised of a voltage dividing circuit, a voltage detecting circuit and a control circuit for outputting a detection result of the detecting circuit as a control signal, as shown in a circuit block diagram of FIG. 2. As the voltage dividing circuit used for the battery voltage detecting circuit, in order to realize a low current consumption at low cost, there is generally employed a polycrystalline silicon film resistor which is high in sheet resistance.
The conventional polycrystalline silicon film resistor is realized by forming an insulating film 2 on a surface of a semiconductor substrate 3, and then forming a polycrystalline silicon film resistor 1 on the insulating film 2, as shown in the structural cross-sectional view of FIG. 3.
In the conventional polycrystalline silicon film resistor shown in FIG. 3, the semiconductor substrate 3 needs to provide the highest potential in the case where the semiconductor substrate 3 is of an n-type and the semiconductor substrate 3 needs to provide the lowest potential in the case where the semiconductor substrate 3 is of a p-type. Therefore, in the battery voltage detecting circuit shown in FIG. 2, a VDD terminal voltage, which is a power supply for the battery voltage detecting circuit, or a VSS terminal voltage, which is a common of the battery voltage detecting circuit is applied to a substrate voltage supply terminal 4, respectively. However, in the polycrystalline silicon film resistor shown in FIG. 3, a parasitic capacity 8 is added between the polycrystalline silicon film resistor 1 and the semiconductor substrate 3 through the insulating film 2. Therefore, in the battery voltage detecting circuit shown in FIG. 2, even if voltages across the batteries 2 and 22 are constant, when a voltage variation occurs at the VDD terminal, which is a power supply, or a VSS terminal, which is a common, the output voltages of the voltage dividing circuits 23 and 24 each of which is comprised of a polycrystalline silicon film resistor vary due to the parasitic capacity 8, and the voltage detecting circuits 25 and 26 react with the varied output voltages, resulting in a problem in that false detection or false release occurs.
Therefore, in order to solve the above-mentioned problems with the related art, according to the present invention, there is provided a polycrystalline silicon film resistor used in a voltage dividing circuit for a battery voltage detecting circuit. The resistor is provided with a well, which is a layer insulated from a semiconductor substrate, disposed on the semiconductor substrate at a side opposite to the polycrystalline silicon film resistor with an insulating film interposed therebetween. In addition, the potential of the well is set to be the same potential as that of the polycrystalline silicon film resistor or a stable potential. As a result, no parasitic capacity is disposed between the semiconductor substrate and the polycrystalline silicon film resistor, to thereby solve a problem in that false detection or false release is led by a voltage variation at a VDD terminal which is a power supply or a VSS terminal which is a common.